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2684ddd72e
Ideally only those transform passes that run at -O0 remain enabled, in reality we get as close as we reasonably can. Passes are responsible for disabling themselves, it's not the job of the pass manager to do it for them. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200892 91177308-0d34-0410-b5e6-96231b3b80d8
101 lines
3.3 KiB
C++
101 lines
3.3 KiB
C++
//===- DCE.cpp - Code to perform dead code elimination --------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements the Aggressive Dead Code Elimination pass. This pass
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// optimistically assumes that all instructions are dead until proven otherwise,
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// allowing it to eliminate dead computations that other DCE passes do not
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// catch, particularly involving loop computations.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "adce"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/ADT/DepthFirstIterator.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/IR/BasicBlock.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/InstIterator.h"
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using namespace llvm;
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STATISTIC(NumRemoved, "Number of instructions removed");
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namespace {
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struct ADCE : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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ADCE() : FunctionPass(ID) {
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initializeADCEPass(*PassRegistry::getPassRegistry());
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}
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virtual bool runOnFunction(Function& F);
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virtual void getAnalysisUsage(AnalysisUsage& AU) const {
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AU.setPreservesCFG();
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}
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};
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}
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char ADCE::ID = 0;
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INITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false)
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bool ADCE::runOnFunction(Function& F) {
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if (skipOptnoneFunction(F))
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return false;
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SmallPtrSet<Instruction*, 128> alive;
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SmallVector<Instruction*, 128> worklist;
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// Collect the set of "root" instructions that are known live.
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for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
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if (isa<TerminatorInst>(I.getInstructionIterator()) ||
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isa<DbgInfoIntrinsic>(I.getInstructionIterator()) ||
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isa<LandingPadInst>(I.getInstructionIterator()) ||
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I->mayHaveSideEffects()) {
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alive.insert(I.getInstructionIterator());
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worklist.push_back(I.getInstructionIterator());
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}
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// Propagate liveness backwards to operands.
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while (!worklist.empty()) {
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Instruction* curr = worklist.pop_back_val();
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for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
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OI != OE; ++OI)
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if (Instruction* Inst = dyn_cast<Instruction>(OI))
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if (alive.insert(Inst))
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worklist.push_back(Inst);
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}
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// The inverse of the live set is the dead set. These are those instructions
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// which have no side effects and do not influence the control flow or return
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// value of the function, and may therefore be deleted safely.
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// NOTE: We reuse the worklist vector here for memory efficiency.
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for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
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if (!alive.count(I.getInstructionIterator())) {
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worklist.push_back(I.getInstructionIterator());
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I->dropAllReferences();
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}
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for (SmallVectorImpl<Instruction *>::iterator I = worklist.begin(),
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E = worklist.end(); I != E; ++I) {
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++NumRemoved;
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(*I)->eraseFromParent();
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}
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return !worklist.empty();
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}
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FunctionPass *llvm::createAggressiveDCEPass() {
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return new ADCE();
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}
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